The present invention relates to an amplifier, and more particularly concerns a high output, low power consumption amplifier.
Also, the present invention relates to a wideband amplifier, and more particularly concerns a high-amplitude, wideband amplifier useful in a picture tube drive circuit which needs a high amplitude output.
The present invention relates to an amplifier applicable to a signal processor useful in controlling a video signal for driving a display apparatus, such as a CRT display.
As the display apparatus is made to have higher resolution in recent years, the picture tube drive circuit tends to have wider frequency band, particularly in a computer display for CAD and CAM which requires a bandwidth as wide as 50 to 300 MHz. The voltage amplitude of the drive signal needed is around 30 V for a monochrome picture tube or as high as around 50 V for a color picture tube. The voltage amplitude is being made higher with current demand for larger display size.
As a result, it is a problem that the power consumption of the drive circuit is increased, resulting in that the circuit components are large sized and heavy weight. In connection with that problem, FIG. 2 depicts a circuit diagram illustrating a capacitive load drive circuit for a prior picture tube or CRT which is described in the Japanese Patent Application Publication No. 57-20724.
The prior capacitive load drive circuit in FIG. 2 operates as follows. A wideband signal fed from a signal source 1 to an input pin 2 is divided into a low-frequency component and high frequency component before being amplified to drive a capacitive load 6. The low-frequency component is amplified by a parallel feedback amplifier circuit formed of a transistor 25 having a feedback path of an input resistor 27, a feedback resistor 7, and frequency response compensation capacitor 28, with a temperature drift and a distortion suppressed. A collector current of a transistor 4 forming a bias constant-current circuit can be suppressed to suppress the power consumption of the amplifier circuit. The high-frequency component is amplified by a series feedback amplifier circuit formed of a transistor 26 having a feedback resistor 31 and a peaking capacitor 32. Both frequency components are composed at an emitter of a grounded-base transistor 3 before being fed out to an output pin 5.
The prior technique described above involves a problem that the wideband signal cannot be amplified to a sufficiently high signal amplitude. That is, if the prior capacitive load drive circuit in FIG. 2 is used to amplify the high-frequency signal to a high amplitude, there is often a side effect wherein the peaking capacitor 32, with the circuit power consumption suppressed, cuts off the transistor 26, resulting in insufficient output amplitude. Further detailed description is given below.
If the input signal falls down, the peaking capacitor 32 has to be discharged to make the voltage waveform at the emitter of the transistor follow the input signal. A maximum discharge current of the peaking capacitor 32, however, is suppressed to a bias current of the transistor. Therefore, in the state that the bias current of the transistor 26 is suppressed to suppress the circuit power consumption if the input signal of high amplitude falls down in a very short transition time, the peaking capacitor 32 cannot be fully discharged, causing the transistor 26 to be cut off.
Also, it is another problem that the frequency response of the feedback system of the prior technique affects the characteristics of the amplifier circuit. For example, a phase delay caused in the feedback network sometimes affects stability of the amplifier circuit too much to fully secure the frequency band. Also, if the frequency band of the feedback network is not fully secured, a shoot caused in the transient response of the amplifier circuit is too much to fully secure the frequency band for the amplifier circuit as described above.
Further, it is still another problem that a load effect of the feedback network sometimes deteriorates the high-amplitude, wideband output capability of the amplifier circuit. In FIG. 2, also, parasitic capacitance and parasitic inductance of the feedback circuit elements, including the input resistor 27, the feedback resistor 7, the frequency response compensation capacitor 28, and the transistor 25 deteriorate the characteristics of the amplifier circuit. Moreover, it is still another problem that the frequency response compensation capacitor 28 makes narrower the frequency band at the time of high amplitude output as it is loaded on the amplifier circuit, although it is used to improve the transient response characteristic of the amplifier circuit.
As the display apparatus is made to have finer definition in recent years, the picture tube drive circuit tends to have higher amplitude and wider frequency band.
The following describes an example of the amplifier used as the picture tube drive circuit described in the Japanese Patent Application Laid-Open No. 60-5693.
FIG. 49 depicts a circuit diagram illustrating the prior amplifier used as the picture tube drive circuit. The amplifier used as the picture tube drive circuit, as shown in FIG. 49, has a cascade amplifier formed of a grounded-emitter transistor 310 and a grounded-base transistor 311. The transistor 310 has a video signal input thereto. The transistor 310 also has at an emitter thereof an emitter peaking circuit 312 for increasing a signal gain of the picture tube drive circuit at a high-frequency range of the video signal. The transistor 311 has at a collector thereof an output resistor 313. The output resistor 313 also is connected in series with a parallel peaking circuit coil 314 for increasing the signal gain of the picture tube drive circuit at the high-frequency range of the video signal. An output signal V.sub.OUT is fed out of a collector pin of the transistor 311 before being connected to the picture tube.
The amplifier constructed as described above can have a wide band because of the cascade construction and the parallel peaking. In fact, however, as shown in FIG. 50 which is a circuit equivalent to the one in FIG. 49, the transistor 311 has some parasitic capacitors amounting to a total parasitic capacitance C.sub.S added onto on the collector. The parasitic capacitors include a load capacitor 315 of capacitance C.sub.L, a parasitic capacitor 316 of capacitance C.sub.C at the collector pin of the transistor 311, a parasitic capacitor 317 of capacitance C.sub.R of the output resistor 313 of resistance R.sub.L, a wiring capacitor 318 of capacitance C.sub.P, and an output capacitor 319 of capacitance C.sub.Ob of the transistor 311.
Let f.sub.BH denote an basic frequency band of the amplifier with no peaking made. The basic frequency band f.sub.BH is given by ##EQU1## The total parasitic capacitance C.sub.S is given by EQU C.sub.S =C.sub.C +C.sub.P +C.sub.R +C.sub.L +C.sub.Ob ( 2)
It can be seen from Eq. (1) that the basic frequency band f.sub.BH is in inverse proportion to C.sub.S and R.sub.L.
In the prior technique, C.sub.S is too high to make wider the frequency ban of the amplifier. If the frequency ban of the amplifier is forcibly made wider with C.sub.S being too high, the value of R.sub.L has to be reduced. As a result, it is a problem that the power consumption of the amplifier is increased.
FIG. 69 depicts a circuit diagram illustrating a prior video amplifier for use in a color CRT display. In the prior circuit shown in FIG. 69, three color video signals R(red), G(green), and B(blue) are fed from the respective signal sources 604R, 604G, and 604B through the respective video signal processor circuits 615R, 615G, and 615B to the respective cathodes 603R, 603G, and 603B of a CRT 604. The video signal processor circuits 615R, 615G, and 615B must have wide frequency band and large output power characteristics, respectively.
The following typically describes operation of the R color circuit 615R only since the other color circuits are identical. An input signal voltage Vir is applied to a base of a grounded emitter transistor amplifier 606R and is inverted and amplified at a collector thereof before being fed out as an output signal voltage Vor. The output signal is applied to the cathode 603B of the CRT 604.
An operating point of the output signal voltage Vor can be adjusted with a cut-off adjustment variable resistor 608R. A voltage gain can be adjusted with a drive adjustment variable resistor 609R. An adjustable range of the operating point of the output signal voltage Vor can be limited with a resistor 607R. A resistor 610R, like the resistor 607R, limits the voltage gain adjustable range.
A prior white balance adjustment can be made by repeating cut-off adjustments of the primary color circuits and drive adjustments of at least two primary color video amplifier circuits.
The prior white balance adjustment process is described below by reference to FIG. 70 which shows input-output characteristic curves of the video amplifier circuit. In the input-output characteristic graph in the figure, Vi denotes the input signal voltage on the axis of abscissas, and Voo denotes the output signal voltage on the axis of ordinates. A solid straight line 650 in the graph is an input-output characteristic with a target white balance secured.
In FIG. 70, assume the white balance to be in a state that the input signal voltage is at values Vic and Vid, which can be achieved with the cut-off adjustment and the drive adjustment, respectively. Also, assume an initial state of the video amplifier circuit is of a characteristic indicated by a broken line 651. With the first cut-off adjustment, the characteristic is moved, or chiefly level-shifted, to the output voltage Voo, or the one indicated by a broken line 653, as shown by an arrow 652.
With the next drive adjustment, a voltage gain adjustment indicated by an arrow 654 is made to change the characteristic to the one indicated by a broken line 655. This ends the first white balance adjustment. However, it is a problem that as can be seen by an arrow 656, the drive adjustment adversely deviates the preceding cut-off adjustment.
Therefore, the prior video amplifier circuit involves such a problem that the cut-off adjustment and the drive adjustment interfere with each other. The white balance adjustment cannot be completed unless the adjustments are repeated.
Also, in the above-described prior technique, any of the video signal processor circuits 615R, 615G, and 615B has to have a wide range of the signal voltage, including the dc component input thereto. This means that any of the video signal processor circuits 615R, 615G, and 615B has to have a wide input dynamic range. However, it is another problem that with the input dynamic range enlarged, the output dynamic range and the power supply voltage must be made high, so that the power consumption of the circuit is increased.
Further, in the above-described prior technique, if a white color temperature displayed on the picture tube (CRT) 604 is varied by a user, drive adjustment variable resistors for the primary color circuits are allowed for the user to adjust. However, when the user who has no measuring instruments is allowed to do the drive adjustment that is the same as the procedures as in factory, it may happen that the output amplitude of any of the primary color signal circuit becomes too high. This results in a deterioration of the linearity of the video circuit and the picture streaks. On the contrary, it may happen that the brightness is made too low. Therefore, it is still a further problem that the brightness is changed with the color temperature varied and further, the white balance is lost.